Late Miocene stable isotope stratigraphy of SE Atlantic ODP Site 1085: Relation to Messinian events

High-resolution benthic oxygen isotope and XRF (Fe and Ca) records from Site 1085 drilled in the Mid-Cape basin (ODP Leg 175) are used to investigate global climate changes during the Late Miocene in relation to Messinian geological events. The cyclic fluctuations of the time series at Site 1085 enable us to establish a reliable chronology for the time interval 7.3–4.7 Ma. Spectral analysis of the δ¹⁸O record indicates that the 41-kyr period of orbital obliquity dominates the Late Miocene record. A global climate record was extracted from the oxygen isotopic composition of benthic foraminifera. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The time interval 7.3–6.25 Ma characterized by low-amplitude δ¹⁸O variations is followed by a period marked by maximum in the δ¹⁸O values (6.25–5.57 Ma). At about 5.56 Ma, a rapid decrease in δ¹⁸O values is documented that may reflect a warming of deep-water temperature associated with a global warming period. Comparison between the timing of the oceanic isotope events and the chronology of the Mediterranean Salinity Crisis suggest that global eustatic processes were not essential in the Mediterranean Salinity Crisis history. From our data, we infer that the global warmth documented in the Early/mid-Pliocene probably started during the Late Miocene (at 5.55 Ma). At the same time, the onset of evaporite deposition in the central basin of the Mediterranean Sea took place. Sharp changes in the sedimentation rates, mainly driven by terrigenous input at this site, are observed during the Messinian Stage.     

Energy Exchange between Jets of the Antarctic Circumpolar Current and Synoptic Eddies in the Drake Passage and Scotia Sea

We study the energy exchange between jets of the Antarctic Circumpolar Current (ACC) and synoptic eddies generated by them in the surface layer of the ocean in the Drake Passage and Scotia Sea based on 22-year-long satellite altimetry time series from the French CLS Agency (DT Global–MADT–Upd product, http://www.aviso.altimetry.fr) under the assumption, based on observations, that each of the jets is confined between two fixed contour lines of the absolute dynamic topography of the ocean. We calculate and analyze the 22-year evolution of the kinetic energy of each ACC jet and cyclonic and anticyclonic eddies generated by it. We demonstrate the fundamental dependence of fluctuations in jet energy on the phase of their meander and eddy formation, as well as their back absorption by jets. We calculate the mean and extreme energetic characteristics of jets and eddies and compare the jets in terms of the intensity of the generated eddies.     

Seasonal cycle of circulation in the Antarctic Peninsula and the off-shelf transport of shelf waters into southern Drake Passage and Scotia Sea

The seasonal cycle of circulation and transport in the Antarctic Peninsula shelf region is investigated using a high-resolution (∼2 km) regional model based on the Regional Oceanic Modeling System (ROMS). The model also includes a naturally occurring tracer with a strong source over the shelf (radium isotope ²²⁸Ra, t_1/2=5.8 years) to investigate the sediment Fe input and its transport. The model is spun-up for three years using climatological boundary and surface forcing and then run for the 2004–2006 period using realistic forcing. Model results suggest a persistent and coherent circulation system throughout the year consisting of several major components that converge water masses from various sources toward Elephant Island. These currents are largely in geostrophic balance, driven by surface winds, topographic steering, and large-scale forcing. Strong off-shelf transport of the Fe-rich shelf waters takes place over the northeastern shelf/slope of Elephant Island, driven by a combination of topographic steering, extension of shelf currents, and strong horizontal mixing between the ACC and shelf waters. These results are generally consistent with recent and historical observational studies. Both the shelf circulation and off-shelf transport show a significant seasonality, mainly due to the seasonal changes of surface winds and large-scale circulation. Modeled and observed distributions of ²²⁸Ra suggest that a majority of Fe-rich upper layer waters exported off-shelf around Elephant Island are carried by the shelfbreak current and the Bransfield Strait Current from the shallow sills between Gerlache Strait and Livingston Island, and northern shelf of the South Shetland Islands, where strong winter mixing supplies much of the sediment derived nutrients (including Fe) input to the surface layer.     

Coupling of biological and physical regimes across the Antarctic Polar Front as reflected by nitrogen production and recycling

Oceanographic samples were collected across the Antarctic Polar Front (APF) region in the vicinity of 60°S, 170°W during the US JGOFS program from December 1997 to March 1998. This paper reports the uptake rates of new (nitrate) and regenerated (ammonium and urea) nitrogen measured by ¹⁵N tracer techniques together with the levels of ammonium, urea-N and dissolved free amino acids (DFAAs) during December and mid-February–March. The APF was an important biological boundary, and in December rates of new (nitrate) uptake were greatest south of the APF, exceeding 10 mmol m⁻² d⁻¹ near the retreating ice edge. In February, nitrate uptake rates were an order of magnitude lower. Rates of ammonium uptake in both periods were greater in the warmer water north of the front. Nitrogen f-ratios varied from 0.50 to less than 0.05, with larger values associated with the >5 μm size fraction at the ice edge and generally lower values north of the APF. Urea was an important nitrogen source north of the APF, and lowered f-ratios there by 22% on average when included as part of total nitrogen uptake. Urea uptake was less important south of the APF. Ammonium concentrations increased dramatically south of the APF later in the season, suggesting a system dominated by regeneration. Seasonal changes in the concentrations of regenerated organic compounds such as urea and DFAAs were less obvious, although DFAAs exhibited consistent maxima in the high flow regions of the APF. A mass balance based of ammonium fluxes suggests that nitrification was significant at locations south of the APF in February. In these nitrate-replete waters, light/mixing conditions in the surface water (the Sverdrup criterion) accounted for over 50% of the variance in nitrate uptake rates. The stability responsible for higher new production south of the APF is due both to the separation of this region from the maximum zonal wind field to the north as well as to melt-water contribution from the retreating ice field. Estimated new production and exportable carbon production exceeded 500 mmol nitrate m⁻² yr⁻¹ and 40 g C m⁻² yr⁻¹, respectively, south of the APF. Thus, new production in the marginal ice zone of the Southern Ocean rivals that in coastal systems and indicates that this is an important region for export production.     

晚第四纪南海北部陆坡沉积物常量元素比值对气候变冷事件的指示意义

南海北部陆坡柱状沉积物的常量元素统计分析表明, 沉积物中的Ti、Al、Fe、K、Mn、Mg代表了陆源元素组合;而消除粒度效应的陆源元素比值Al/Ti、Fe/Ti和K/Ti等的变化曲线与浮游有孔虫氧同位素曲线对比表明, 在气候变冷事件(Heinrich事件、Younger Dryas事件、PME斜室普林虫低值等事件)期间, 陆源常量元素比值明显降低。这一对应特征, 一方面体现了气候变冷事件的发生对陆源区化学风化强度的制约, 另一方面也说明与指示长时间尺度气候相似, 陆源元素比值对短时间尺度的气候变化也具有明显的指示作用。     

The influence of shelf processes in delivering dissolved iron to the HNLC waters of the Drake Passage,Antarctica

Dissolved trace element distributions near Elephant Island in the Drake Passage show extremely high levels of dissolved Fe and Mn in waters above the shelf. The entrainment of this enriched shelf water by the Fe-poor Antarctic Circumpolar Current (ACC) as it passes through the Shackleton Gap delivers an estimated 2.8×10⁶ mol yr⁻¹ dissolved Fe to the offshore waters of the Drake Passage. The magnitude and spatial distribution of dissolved Fe, Mn and Al over the shelf are consistent with a diagenetically produced sedimentary source, but are inconsistent with eolian or upwelling sources. The systematics of the Mn and Fe concentrations suggest that there are two distinct sources of dissolved Fe to the surface waters of this region. The highest Fe concentrations are associated with Bransfield Strait water, which can be identified by its characteristic temperature and salinity (T/S) properties both inside the Bransfield Strait and in the Bransfield Current outflow between Elephant and Clarence Islands. Most of the shelf area is dominated by a second water type with T/S properties that are typical of modified Antarctic Surface Water, which while also enriched has a lower Fe:Mn ratio.The predominantly linear relationships between the Fe and Mn concentrations at the stations in each of these water mass types suggest that the distribution of these elements is largely controlled by physical mixing processes and that biological removal of Fe on the shelf, while certainly occurring, is limited, perhaps as a result of rapid physical flushing processes and relatively slow biological growth rates. The consequent export of large quantities of this shelf-derived Fe into the ACC is likely responsible for the extensive regions of enhanced primary production seen in satellite imagery downstream of the Drake Passage.     

Seismic structure and tectonics of the Shackleton Fracture Zone (Drake Passage,Scotia Sea)

The structural framework of the southern part of the Shackleton Fracture Zone has been investigated through the analysis of a 130-km-long multichannel seismic reflection profile acquired orthogonally to the fracture zone near 60° S. The Shackleton Fracture Zone is a 800-km-long, mostly rectilinear and pronounced bathymetric lineation joining the westernmost South Scotia Ridge to southern South America south of Cape Horn, separating the western Scotia Sea plate from the Antarctic plate. Conventional processing applied to the seismic data outlines the main structures of the Shackleton Fracture Zone, but only the use of enhanced techniques, such as accurate velocity analyses and pre-stack depth migration, provides a good definition of the acoustic basement and the architecture of the sedimentary sequences. In particular, a strong and mostly continuous reflector found at about 8.0 s two-way traveltime is very clear across the entire section and is interpreted as the Moho discontinuity. Data show a complex system of troughs developed along the eastern flank of the crustal ridge, containing tilted and rotated blocks, and the presence of a prominent listric normal fault developed within the oceanic crust. Positive flower structures developed within the oceanic basement indicate strike-slip tectonism and partial reactivation of pre-existing faults. Present-day tectonic activity is found mostly in correspondence to the relief, whereas fault-induced deformation is negligible across the entire trough system. This indicates that the E–W-directed stress regime present in the Drake Passage region is mainly dissipated along a narrow zone within the Shackleton Ridge axis. A reappraisal of all available magnetic anomaly identifications in the western Scotia Sea and in the former Phoenix plate, in conjunction with new magnetic profiles acquired to the east of the Shackleton Fracture Zone off the Tierra del Fuego continental margin, has allowed us to propose a simple reconstruction of Shackleton Fracture Zone development in the general context of the Drake Passage opening.     

Phytoplankton pigment distribution in relation to silicic acid, iron and the physical structure across the Antarctic Polar Front, 170°W, during austral summer

In order to study the factors controlling the phytoplankton distribution across the Antarctic Polar Frontal Region (PFR), surface pigment samples were collected during austral summer (January/February 1998) near 170°W. Both the Polar Front (PF) and the Southern Antarctic Circumpolar Current Front (SACCF) were regions of enhanced accumulation of phytoplankton pigments. The mesoscale survey across the PF revealed two distinct phytoplankton assemblages on either side of the front. The phytoplankton community was dominated by diatoms south of the PF and by nanoflagellates (primarily by prymnesiophytes) to the north. Surprisingly, chlorophyll a concentrations did not correlate with mixed-layer depths. However, an increase of the dominance of diatoms over prymnesiophytes was observed with decreasing mixed-layer depths. Despite this relationship, we conclude that the average light availability in the mixed layer was not an important factor influencing the shift in phytoplankton composition across the PF. Although no correlation was found between the surface distribution of the major phytoplankton taxa and dissolved iron or silicic acid concentrations, the location of the strongest vertical gradient in silicic acid and iron concentration coincides with the maximum abundance of diatoms. We conclude that the difference in taxonomic composition is a result of increased silicic acid and iron flux to the upper mixed layer as a result of the increased vertical gradient of these key nutrients south of the front.     

南极布兰斯菲尔德海峡区域重力场特征及异常分析

南极布兰斯菲尔德海峡及周边区域是南极大陆火山、地震等新构造活动最活跃的地区,与南设得兰海沟、南设得兰群岛一同构成南极大陆边缘现存唯一的"沟-弧-盆"构造体系。本文基于"雪龙"船第28、第30航次实测数据及两个航次的国际共享资料,利用均衡改正数据处理方法获得布兰斯菲尔德海峡的莫霍面深度及其分布规律,分析深部构造-断裂的区域分布及其重力异常特征等。布兰斯菲尔德海峡内的空间重力异常呈条带状分布,走向总体与地形相近,布格重力异常则由两侧向中间升高,大致在坡折处形成异常场值为100×10-5 m/s2的分界线,在中央次海盆和东部次海盆海山处形成两个异常高值圈闭,异常值最高为150×10-5 m/s2。莫霍面深度以弧后扩张中心为最低值,向南设得兰群岛和南极半岛两个方向递增,深度从12 km递增至陆坡位置的24 km。     

A record of fluctuating bottom currents on the slopes west of the Porcupine Bank,offshore Ireland — implications for Late Quaternary climate forcing

Both local and regional controls on slope sedimentation west of Porcupine Bank are assessed using an array of 25 gravity cores, integrated with shallow seismic, TOBI side-scan and high-resolution bathymetry data. The cores were retrieved from an area of smooth, distally steepened slope (between 52° and 53°N) in water depths of 950 to 2750 m. The slope here is unmodified by gravity failures and is swept by bottom currents that flow from S to N along the margin. The cores reveal a coherent shallow stratigraphy that can be traced along and between transects at upper-, mid- and lower-slope levels. AMS ¹⁴C dating, oxygen-isotopes and carbonate profiles suggest the cored record could extend as far back as 500 ka in the longest cores, with most cores providing details of the slope response to the last interglacial, last glacial and Holocene forcing. The facies indicate deposition was dominated by a combination of bottom currents, ice-rafting and hemipelagic settling, with carbonate-prone deposits during interglacials, and siliciclastic deposits during glacials. Inferred contourites imply that strong currents operated during interglacials, with weaker current reworking during glacial conditions. A pair of erosion surfaces record significant mid- and upper-slope scouring during Marine Isotope Stage (MIS 3) and in the Early Holocene. The lateral facies distribution implies stronger currents at shallower levels on the slope, although there is evidence that the core of the current migrated up and down the slope, and that sand might locally have spilt down-slope. The bathymetry influenced both the wider geometry of the condensed contourite sheet and the local thickness and facies variation across the slope. A significant result of the study is the identification of a pair of thin sand–mud contourite couplets that record enhanced bottom-current reworking corresponding to periods of interstadial warming during MIS 3. The couplets can be correlated to the terrestrial records onshore Ireland and imply that the NE Atlantic margin oceanographic and onshore climate records are strongly coupled at interstadial level.     

The seasonal succession of zooplankton in the Southern Ocean south of Australia,part II: The Sub-Antarctic to Polar Frontal Zones

Between October 2001 and March 2002 six transects were completed at monthly intervals in the Sub-Antarctic Zone (SAZ) and Inter-Sub-Antarctic Front Zone (ISAFZ)/Polar Frontal Zone (PFZ) in the Southern Ocean south of Australia. Zooplankton were collected with a Continuous Plankton Recorder and NORPAC net and multivariate analysis was used to analyse the seasonal succession of communities. Despite strong, seasonally consistent, biogeographic differences between the SAZ and ISAFZ/PFZ, community structure in all zones was dominated by a suite of common taxa. These included the ubiquitous Oithona similis, foraminiferans and appendicularians (Core taxa), occurring in >97% of samples and contributing an average of 75% to total sample abundance, and Calanus simillimus, Rhincalanus gigas, Ctenocalanus citer, Clausocalanus brevipes, Clausocalanus laticeps, Oithona frigida, Limacina spp. and chaetognaths (Summer taxa), present in >57% of samples and occurring at seasonally high densities. Because of the dominance of the Core and Summer taxa, the seasonal succession was most clearly evident as a change in zooplankton densities. In October densities averaged <15 ind m⁻³, rising to 52 ind m⁻³ (max=92 ind m⁻³) in November, and subsequently increasing slowly through to January (ave=115 ind m⁻³; max=255 ind m⁻³). Densities peaked abruptly in February (ave=634 ind m⁻³; max=1593 ind m⁻³), and remained relatively high in March (ave=193 ind m⁻³; max=789 ind m⁻³). A latitudinal lag in seasonal development was observed with peak densities occurring first in the SAZ (February) and then in the ISAFZ/PFZ (March). The seasonal community succession was strongly influenced by species population cycles. The role of zooplankton in biogeochemical cycling in the SAZ and ISAFZ/PFZ was discussed in the light of past sediment trap data collected from the study area.     

晚第四纪南极阿蒙森海扇区冰盖与古生产力旋回变化

本文通过对阿蒙森海西北部陆隆AMS01岩心沉积物颜色、粒度及地球化学等多种指标分析,重建了该地区氧同位素9期(MIS9,大约距今34万年)以来冰盖与古生产力演变历史,结果表明:(1)岩心沉积物粒度与古生产力替代指标表现出明显的冰期–间冰期旋回变化特征;(2)MIS9、MIS7和MIS5等间冰期沉积速率较小,沉积物呈褐色,冰筏碎屑含量低,生源组分含量高,反映出该时期阿蒙森海地区气候温暖,冰盖发生了大规模退缩,冰盖–冰架–冰山等陆源冰对沉积物的影响减弱,海冰覆盖减弱,有利于浮游植物的生长和繁殖;(3)MIS8c、MIS8a、MIS6、MIS2等冰期沉积速率大,沉积物呈灰色,沉积物随之变粗,冰筏碎屑含量高,生源组分含量低,说明该时期冰盖大幅扩张,陆隆区成为近冰盖/冰架沉积环境,海冰和冰山密集,海洋生产力显著降低;(4)冰期、间冰期内,冰盖与古生产力也有不同程度的波动;特别是MIS8b期发育浅褐色间冰阶沉积,冰筏碎屑含量低,生产力水平与间冰期基本持平,说明阿蒙森海地区冰盖、海洋对气候变化的响应比东南极地区敏感。     

Late Pliocene to Quaternary sedimentary facies and stratigraphy of shallow-water contourite deposits in the Hupo Basin,East Sea of Korea

Marine Geophysical Research - This study entails the characterization of the depositional environment of the Hupo Basin shelf. By means of sedimentary structure analysis, grain size, textures,...     

黄东海及邻近陆架晚第四纪层序地层

概述了近年来黄东海晚第四纪地层的最新研究进展，总结了末次冰期最盛期以来的层序地层模式，即：本区域的海平面呈阶梯式变化，可区分为低水位、海侵、高水位3大沉积体系域。     

A new TOC,Rock-Eval,and carbon isotope record of Lower Jurassic source rocks from the Slyne Basin,offshore Ireland

This paper presents an integrated assessment of total organic carbon, Rock-Eval pyrolysis, carbonate δ¹³C and δ¹⁸O, and total organic carbon δ¹³C record of Upper Sinemurian–Aalenian age from the Slyne Basin (well 18/25-1), offshore Ireland. Total organic carbon (up to 5.2 wt%) and Rock-Eval pyrolysis data highlights the Portree Shale Formation Equivalent (Lower–?Middle Toarcian) as a mature (T_max ∼440 °C) source rock dominated by type II kerogens. This interval is more than 90 m thick.In a previously unexplored paleogeographical domain regarding stable carbon isotopes, the δ¹³C record shows positive values during the Upper Sinemurian, a negative trend followed by a minor positive trend during the uppermost Sinemurian–Lower Pliensbachian, a negative trend during the lowermost Lower Toarcian, and then the return to more positive δ¹³C values. Despite the biostratigraphic uncertainty, the coarse resolution of the dataset, and the complex assessment of diagenetic effects, we suggest that the δ¹³C record of the studied well records the Raricostatum Zone positive CIE, the Sinemurian-Pliensbachian Boundary Event, and the negative CIE associated with the T-OAE. The T-OAE positive CIE is not clearly defined in the obtained dataset. The slight post T-OAE negative CIE recovery to more positive values and the continuation of a relatively high organic content is coeval with a regional organic matter preservation interval observed in several Northern-European locations.These new datasets and their interpretation add to the understanding of the paleoenvironmental dynamics during the Early–?Middle Jurassic in the Northern European domain and opens new possibilities for hydrocarbon exploration in Ireland's offshore.     

Glucose fluxes and concentrations of dissolved combined neutral sugars (polysaccharides) in the Ross Sea and Polar Front Zone, Antarctica

We hypothesized that dissolved carbohydrates would be large components of the labile dissolved organic carbon (DOC) pool and would support much bacterial growth in Antarctic waters, especially the Ross Sea, since previous work had observed extensive phytoplankton blooms with potentially high production rates of carbohydrates in Antarctic seas. These hypotheses were tested on cruises in the Ross Sea and Antarctic Polar Front Zone as part of the US JGOFS program. Concentrations and fluxes of free glucose (the only free sugar detected) were very low, but dissolved polysaccharides appeared to be important components of the DOC pool. Concentrations of dissolved combined neutral sugars increased >3-fold during the phytoplankton bloom in the Ross Sea and were a large fraction (ca. 50%) of the semi-labile fraction of DOC. The relatively high concentrations of dissolved combined neutral sugars, which are thought to be quite labile, appear to explain why DOC accumulated during the phytoplankton bloom was degraded so quickly once the bloom ended. Some of the polysaccharides appeared to be more refractory, however, since dissolved combined neutral sugars were observed in deep waters (>550 m) and in early spring (October) in the Ross Sea, apparently having survived degradation for >8 months. The molecular composition of these refractory polysaccharides differed from that of polysaccharides sampled during the phytoplankton bloom. Fluxes of DOC were low in the Ross Sea compared to standing stocks and fluxes of particulate material, but the DOC that did accumulate during the phytoplankton bloom appeared to be sugar-rich and relatively labile.     

Middle to Late Cenozoic tectonic events in south and central Palawan (Philippines) and their implications to the evolution of the south-eastern margin of South China Sea: Evidence from onshore structural and offshore seismic data

Using recently gathered onland structural and 2D/3D offshore seismic data in south and central Palawan (Philippines), this paper presents a new perspective in unraveling the Cenozoic tectonic history of the southeastern margin of the South China Sea. South and central Palawan are dominated by Mesozoic ophiolites (Palawan Ophiolite), distinct from the primarily continental composition of the north. These ophiolites are emplaced over syn-rift Eocene turbidites (Panas Formation) along thrust structures best preserved in the ophiolite–turbidite contact as well as within the ophiolites. Thrusting is sealed by Early Miocene (∼20 Ma) sediments of the Pagasa Formation (Isugod Formation onland), constraining the younger limit of ophiolite emplacement at end Late Oligocene (∼23 Ma). The onset of ophiolite emplacement at end Eocene is constrained by thrust-related metamorphism of the Eocene turbidites, and post-emplacement underthrusting of Late Oligocene – Early Miocene Nido Limestone. This carbonate underthrusting at end Early Miocene (∼16 Ma) is marked by the deformation of a seismic unit corresponding to the earliest members of the Early – Middle Miocene Pagasa Formation. Within this formation, a tectonic wedge was built within Middle Miocene (from ∼16 Ma to ∼12 Ma), forming a thrust-fold belt called the Pagasa Wedge. Wedge deformation is truncated by the regionally-observed Middle Miocene Unconformity (MMU ∼12 Ma). A localized, post-kinematic extension affects thrust-fold structures, the MMU, and Late Miocene to Early Pliocene carbonates (e.g. Tabon Limestone). This structural set-up suggests a continuous convergent regime affecting the southeastern margin of the South China Sea between end Eocene to end Middle Miocene. The ensuing structures including juxtaposed carbonates, turbidites and shallow marine clastics within thrust-fold belts have become ideal environments for hydrocarbon generation and accumulation. Best developed in the Northwest Borneo Trough area, the intensity of thrust-fold deformation decreases towards the northeast into offshore southwest Palawan.     

A preliminary investigation of the fish food web in the Gironde estuary,France, using dietary and stable isotope analyses

Carbon and Nitrogen stable isotopes and stomach contents analyses were used to investigate an estuarine fish food web and identify the contribution of these two methods to the knowledge and understanding of the food web's structure and its functioning. The nine most abundant fish species during the warm period in the Gironde estuary (southwest France, Europe) are examined. Observation of the stomach contents reflects a variety of feeding modes between fish species that consume a diverse assortment of prey, with limited dietary overlap. Nevertheless, when regarding the whole fish community, few prey species dominate the stomach contents. Nitrogen isotope ratios indicate a high intraspecific variability inducing an interspecific covering of the signatures. However, a tendency to δ¹⁵N enrichment according to the trophic position of the species studied was observed. Fish assemblages show a trend towards enrichment of their carbon isotopic signatures from the upper estuary (−20.8 ± 1.8‰) towards the lower estuary (−18.3 ± 1.6‰). But whatever the capture zone considered, most of the individual δ¹³C values for each fish analysed are comprised between −22 and −16‰. Only few specimens, belonging to migratory amphihaline species, have significantly lighter values.